explicitly enable dynamic agent loading in tests

Prevents lots of warnings like these:

    WARNING: A Java agent has been loaded dynamically (/home/.../
    .gradle/caches/modules-2/files-2.1/net.bytebuddy/byte-buddy-agent/1.14.11/f9cb566608fbac6bc7bf54901a7aa11543a989ee/byte-buddy-agent-1.14.11.jar)
    WARNING: If a serviceability tool is in use, please run with -XX:+EnableDynamicAgentLoading to hide this warning
    WARNING: If a serviceability tool is not in use, please run with -Djdk.instrument.traceUsage for more information
    WARNING: Dynamic loading of agents will be disallowed by default in a future release

build.gradle

@@ -193,6 +193,11 @@ subprojects {
             jvmArgs('--add-opens', 'java.base/sun.nio.ch=ALL-UNNAMED')
             jvmArgs('--add-opens', 'java.base/java.util.zip=ALL-UNNAMED')
         }
+
+        if (buildJavaVersion >= 21) {
+            jvmArgs('-XX:+EnableDynamicAgentLoading')
+        }
+
         testClassesDirs = testing.suites.test.sources.output.classesDirs
         classpath = testing.suites.test.sources.runtimeClasspath